Technical Abstract:
Switzerland is officially free of bovine tuberculosis (OTF) since 1960. A mandatory eradication program had been launched in 1950, herd prevalence at that time accounted for 25%. Since 1980 the control of bovine tuberculosis (bTB) has been reduced to passive abattoir surveillance. Single cases of bTB, partly due to reactivation of human Mycobacterium (M.) bovis infections with subsequent transmission to cattle, have been noticed in the last years.
In Europe, the overall prevalence of bTB is currently slightly increasing. In 2007, 0.53% of cattle herds were bTB positive compared to 0.48% in 2006. Both OTF and non-OTF countries report increases of the proportion of bTB positive cattle herds.
Current bTB eradication and control programs in Europe are facing a range of challenges. Eradication in high prevalence countries like the United Kingdom is expected to take at least 20 years. Wildlife reservoirs both in non-OTF and OTF countries are responsible for reinfections of and spillover to livestock. Whole herd depopulation is becoming less an option due to economic reasons and due to animal welfare concerns. Live animal trade is increasing both at national and international levels. Trade underlies more and more concepts like regionalization, zoning and compartimentalisation. Regarding these tendencies and taking into account chronicity of bTB disease, pre-movement testing is becoming increasingly important as a central tool for eradication and for protection against reintroduction of bTB. Pre-movement testing, however specifically focuses on the infection status in individuals. This illustrates that requirements for diagnostic tests have changed. A high level of reliability to correctly diagnose infected animals is needed. Current screening tests for bTB, however, have been designed to meet demands as herd tests. This illustrates that the modification of existing and/or the development of new diagnostics for bTB might be required. In this presentation we address these issues by summarizing published data on diagnostic tests for bTB in addition with new data.
The tuberculin skin test (TST) constitutes the primary screening test for bTB, either as caudal fold test (CFT), as single cervical intradermal test (CIT) or as comparative cervical test (CCT). Summary of data from international studies indicate 68-96.8% sensitivity and 96-98.8% specificity for CFT, 80-91% sensitivity and 75.5-96.8% specificity for CIT, 55.1-93.5% sensitivity and 88.8-100% specificity for CCT. Recent data from Northern Ireland and a particular region in France (Camargue) indicate a markedly reduced sensitivity of TST: only 59% (CCT, Northern Ireland) respectively 10.6% (CIT, Camargue) of animals with confirmed M. bovis infection were detected.
The interferon gamma (IFN-gamma) assay is used complementary to TST as screening test for bTB. Sensitivity and specificity of the IFN-gamma assay estimated in international studies range from 73.0 to 100% and from 85.0 to 99.6%. The IFN-gamma assay is accepted to be more sensitive compared to skin test. Follow-up studies of TST-negative cattle showed that IFN-gamma positive cattle were at an Odd ratio (OR) of 8.9 to become a TST-reactor at the next testing occasion, respectively IFN-' positive cattle had an 18% risk of reacting TST-positive within one year. In France (Camargue), the IFN-' assay achieved a herd and animal sensitivity of 96 and 66.2%, compared to CCT with only 58 respectively 10.6%. Reduced specificity of the IFN-' assay, however, especially in areas of low bTB prevalence rises concerns. Delayed initiation of culture in a PPD-based assay (24h compared to 8h after blood collection) resulted in a significant improvement of specificity (97% compared to 85%), whereas there was only a modest reduction of sensitivity (from 96% to 90%), which was statistically not significant. Still, the replacement of tuberculins by defined antigens is regarded to have the greatest potential for the improvement of specificity. ESAT-6 and CFP-10 have been shown to be outstanding diagnostic target proteins in the whole blood IFN-gamma assay. Results from a recent field trial in Great Britain indicate that a peptide cocktail composed of peptides from ESAT-6 and CFP-10 (E/C) achieved a sensitivity of 81.3% and specificity of 97.0%, as opposed to 96.7% specificity with the PPD-based IFN-gamma assay. The gain in specificity by the use of E/C may be more striking in regions with a low bTB prevalence close to eradication and in OTF countries. A study performed in Switzerland testing a selected herd with an extraordinarily high percentage of unspecific reactors in the PPD-based assay (specificity of 66.7%) resulted in an E/C-specificity of 97.1%. In recent studies promising antigen combinations for the IFN-gamma assay resulting in improved sensitivity and specificity have been described. Furthermore, shortening pre-culture time before whole blood stimulation with a peptide cocktail derived from defined M. bovis antigens has the potential to increase sensitivity, this however warrants further examination. An "in-tube" or "in-plate" stimulation device allowing rapid stimulation of lymphocytes after blood collection would surely be beneficial to overcome logistical difficulties.
The combined use of the in vivo and in vitro cell mediated immunity assays raises the question whether the IFN-gamma responses are influenced by the TST. The influence of the TST on in vitro IFN-gamma response depends on the location of in vivo application of tuberculins. No significant effect can be observed after CCT. Boost and depression of IFN-gamma production associated with CFT in experimentally infected cattle require further investigation in naturally infected cattle.
Lesion detection, the screening test used for active and passive surveillance of bTB, exhibits a major lack in sensitivity. Indeed, lesions may be localized and small, and therefore easily overlooked during abattoir inspection. A study in Ireland showed that up to 53% of affected cattle may go undetected, we present additional data confirming this low sensitivity.
Demands on diagnostics within bTB eradication programs change along with their progress: At an early stage major focus lies on maximum sensitivity. Later when programs progress towards their completion, highly specific diagnostic tests are needed for economic reasons. Comparison of tests taking into account additional data such as predictive values (the probability that a positive / negative test result reflects a truly infected / non-infected animal) may provide valuable information on the individual test results in different epidemiologic settings.
In conclusion, regarding the fact that some parameters linked with bTB programs are changing, the development of improved diagnostic tests will be important for future eradication of bTB, in line with international commitment to high standard animal health programs.